Rhododendron Dieback

Rhododendrons and azaleas are among the most easily recognizable woody plants along the Blue Ridge Parkway. There are 11 species of rhododendrons and azaleas on the parkway, each with its unique, varied coloration and eye-catching flowers in the spring. However, when these plants become exposed to environmental stressors, they can be highly affected by two major diseases in the southeast: Botryosphaeria dieback and Phytophthora root rot. This has been affecting rhododendron stands and reducing the number of showy blooms in recent years.

What is occurring?

Botryosphaeria dieback—also referred to as Rhodo Dieback or Twig Blight—is the most common Rhododendron disease in the wild. Caused by the fungal plant pathogen, Botryosphaeria dothidea , it usually results in a few characteristic symptoms such as reddish brown discoloration on the underside of the infected branches, sudden leaf wilting and eventual foliage death. It has a world-wide distribution and lives in the soil, affecting many different plant species, including our rhododendrons. The disease is more fatal to older, or drought-damaged plants. Rosebay rhododendron (Rhododendron maximum) is most effected but other species of rhododenron and azaleas are effected as well. Rhododendron are often found as even-aged stands, so many plants in an area can die around the same time. Dieback has also been occurring in surrounding National Forests, as well as in Great Smoky Mountains National Park.

Phytophthora root rot occurs mainly in plant nurseries and is a fungal disease which attacks individual leaves and stems.

What are the potential impacts to visitors and park operations?

The impact will primarily be visual as patches of rhododendron become infected and the plants die. Generally these will be in widely scattered areas and will just be a small portion of the overall landscape. Rhododendron landscaping in developed areas may need to be replanted, but plants in natural areas will be allowed to die and natural regeneration should follow.There are no effective treatments for these funguses. What as the park learned?

Biologists are involved in ongoing research to learn more about these diseases and how they affect native flora. It has been suggested that rhododendrons and azaleas growing outside of their normal ranges—areas higher in elevation with drier slopes—also experience considerable of damage.

Next step, if any:

More research will be necessary to better understand both Botryosphaeria and Phytophthora and their effects on rhododendron populations. Rhododendron dieback and Phytophthora Root Rot are also native diseases which still have an important role in the ecosystem. These plant pathogens also influence the genetic variation of rhododendrons and azaleas, which is essential to their progress and evolution throughout space and time. Biologists do not believe that this disease could lead to extinction.